As supply voltage VDD is reduced with each successive technology node, thermionically-injected FETs (in which electrons or holes are injected into the channel by propagating over, rather than through, an energy barrier) may suffer from decreased gate overdrive VOV (the portion of the gate-source voltage in excess of the threshold voltage Vt). This may be attributed to a fundamental lower bound (60 mV/dec) on the sub-threshold slope SS of thermionically-injected FETs. The bounded sub-threshold slope SS may indicate that, at fixed leakage current levels, the threshold voltage Vt may not scale with supply voltage VDD. The lack of scaling of the threshold voltage Vt can lead to reduced gate overdrive as supply voltage VDD is reduced, and an overall degradation in performance of the thermionically-injected FET. As such, thermionic FETs can provide improved performance for higher-power applications, but may be limited in low-power regimes by the fixed lower bound of the subthreshold slope SS.
In contrast with thermionic FETs, tunnel FETs or TFETs (in which electrons or holes are injected into the channel by propagating through, rather than over, an energy barrier) may not have a fixed lower bound on the sub-threshold slope SS. Since injection into the channel can be governed by tunneling rather than energy sampling from a Fermi-Dirac distribution, the sub-threshold drain current (Id)-gate voltage (Vg) curve can be relatively steeper than that of thermionic FETs. The threshold voltage Vt of the TFET devices can therefore be relatively lower than that of thermionic devices, for the same off-current Ioff. As a result, TFETs may offer improved low-supply voltage VDD performance as compared to thermionic FETs. However, higher-supply voltage VDD performance of TFETs may be more limited, as the tunneling nature of the source-channel injection can limit performance. In other words, tunnel FETs can provide improved performance for lower-frequency applications, but may not be overdriven to sufficient performance levels for higher-frequency applications.